Neutrons are uncharged particles that can travel through matter without ionizing the matter. Because neutrons travel through matter in such a manner, neutrons are difficult to detect directly. Some other evidence of a neutron event must be detected in order to determine its existence. An indirect method detects the results of a neutron event and not the neutron event itself.
Neutron radiation is an unequivocal signature of the presence of transuranic elements associated with nuclear power-generated plutonium and enriched uranium and plutonium derived from the disassembly of nuclear weapons. Both passive and active neutron detection methods have been used in such applications, with the latter involving detection of the secondary fission neutrons induced by a brief pulse of neutrons. The prerequisite for neutron scintillators is the presence of neutron absorbing elements, which is not required for the fabrication of other radiation scintillators such as beta and alpha radiations. Favored isotopes for neutron absorption include 10B, 6Li, 3He, and 235U, all of which have high absorption cross sections for thermal neutrons. In previous work by Wallace et. al. (Nuclear Instruments and Methods in Physics Research A, 2002), a surface barrier detector based on sol-gel technology was reported. Then, Im et al. reported in Applied Physics Letters, March 2004, an approach to neutron scintillator fabrication that employs a room temperature sol-gel processing.
Industries and geological survey agencies who use or need to detect neutrons are interested in the development of new neutron detectors with advantages in efficiency and versatility over the methods in current use. An improved neutron detector technology could even play a role in national security in screening for insecure fissile weapons materials. Currently, solid-state neutron scintillators such as 6Li-doped silica glasses and solid mixtures of 6LiF and ZnS:Ag are prepared by high-temperature methods. Because of the high temperature employed, these materials are very difficult to integrate as films into electronic devices for neutron detection or to cast as large screens. Furthermore, the high temperature methods eliminate the possibility of using organic scintillators because such organic compounds are seldom stable at elevated temperatures. High tritium counting efficiencies based on different types of liquid scintillating cocktails have been reported; however, these liquid scintillators must be handled carefully and are difficult to secure for field applications due to their tendency to leak. Thus, the development of efficient solid-state scintillating materials, which will significantly enhance general capabilities for in situ monitoring and imaging of radioactive contaminants in the environment, is demanded.
Neutron detection for monitoring the dose of thermal neutrons given to patients receiving boron neutron-capture therapy has used lithium-6 and a cerium activator in a glass fiber (M. Bliss et. al., IEEE Trans. Nucl. Sci., 1995). Hiller et. al., in U.S. Pat. No. 5,973,328, issued on Oct. 26, 1999, improve this technique by allowing a cerium-activated glass fiber to be coated with fissionable elements. A wet chemistry method of placing radioactive fissile elements into glass—which in the vitrified state does not pose a hazard—as described in the '328 patent using sol-gel based technology, is a significant benefit. M. Ghioni et. al. (1996) describe an avalanche photodiode implementation for detecting neutron induced ionization and optical pulse detection.
The '328 device introduced sol-gel techniques unique in the art of neutron detection. Sol-gel chemistry was first discovered in the late 1800s. This area of chemistry has received renewed interest when the process was found to be useful in producing monolithic inorganic gels at low temperatures that could be converted to glasses without a high temperature melting process. Brinker et. al., in 1990, provide a comprehensive explanation of sol-gel chemistry. Sheng Dai et. al., in 1996, provide further detail disclosing uranyl-doped sol-gel glasses.
Emission detectors such as microchannel plates, channeltrons, and avalanche photodiodes are commonly used for detecting ultraviolet (UV) light and fissioned charged particles such as electrons or protons. Microchannel plates are commercially available and well known in the art. Typically, a microchannel plate is formed from lead glass having a uniform porous structure of millions of tiny holes or microchannels. Each microchannel functions as a channel electron multiplier, relatively independent of adjacent channels. A thin metal electrode is vacuum deposited on both the input and output surfaces to electrically connect channels in parallel. Microchannel plates can be assembled in stacked series to enhance gain and performance.
The microchannel plates serve to amplify emissions from fissionable material resulting from the bombardment of neutrons. The amplified signal is then detected and recorded. The signal frequency is proportional to the charged particle emissions, which are proportional to the amount of neutrons bombarding the fissionable material.
Channeltrons operate on the same basic principal of amplifying proportional signals emitted from fissionable materials. A channeltron is a horn-shaped continuous dynode structure that is coated on the inside with an electron emissive material. An ion striking the channeltron creates secondary electrons that have an avalanche effect to create more secondary electrons and finally a current pulse.
Typically, due to the exotic materials and sensitivity of the equipment, the neutron detectors currently available are expensive and difficult to maintain. For example, helium-3 is an extremely rare stable isotope which must be separated at considerable expense from the radioactive gas tritium. Furthermore, the use of a gas absorber results in a slower response time than a solid absorber as disclosed herein. The '328 device thus incorporated fissionable material into a sol-gel composition in combination with an emission detector.